DESCRIPTION: Once a protein is implicated in cancer, it is next important to define its role in cancer and how it functions in cancer. Skp2 is an F-box protein of the SCF(Skp2) ubiquitin ligase complex targeting p27Kip1 for ubiquitination. In clinical cancer samples, reduction of p27 and increase in Skp2 are frequent and generally associated events. The conventional view at present is that Skp2 functions as an oncoprotein by targeting p27 for ubiquitination and degradation and, accordingly, the function of SCF(Skp2) to ubiquitinate p27 has been the target of drug development efforts. More recent studies however have indicated the needs to more carefully study the significance of p27 ubiquitination by SCF(Skp2) in tumorigenesis. We have recently determined that Skp2 binds to the N-terminus of cyclin A to directly protect it from inhibition by p27 family CKIs; and a blocking peptide for Skp2-cyclin A interaction can selectively induce cancer cell death in culture. These findings led us to hypothesize that binding to cyclin A is an important function of Skp2 in cancer. In this project, we propose to use our recent discovery that Skp2 plays an essential role in Rb deficiency-induced pituitary and thyroid tumorigenesis to determine the roles of SCF(Skp2)-mediated p27 ubiquitination and binding to cyclin A in Skp2 function in this tumor model. ENU-induced tumorigenesis will be used to extend these studies to a tumor model involving multiple organs. Further, we will use mouse embryo and biochemical studies as well as genetic studies to determine how Skp2 function and Rb function integrate to sustain Rb-deficient cells for tumorigenesis in the presence of Skp2, and to eliminate Rb-deficient cells when Skp2 is inactivated. Results from these studies will elucidate the roles and functional mechanisms of the tumor suppressor Rb, oncogene Skp2, and their relationship in normal physiology and in cancer to reveal therapeutic targets.